fix: skip validator duties while syncing (lambdaclass#418)

## 🗒️ Description / Motivation

- Prevents validators from proposing blocks or producing attestations
while their local view is syncing.
- Signing with a stale local view can place proposals and attestations
on a chain the network has already left.
- Builds on the leanEthereum/leanSpec#708 sync heuristic added in the
preceding PR (lambdaclass#417 ).

## What Changed

- Gate scheduled block proposals using the node’s sync status.
- Report `has_proposal = false` to the store when a proposal is skipped.
- Gate attestation production at interval 1.
- Log skipped proposal and attestation duties.
- Keep block processing, fork choice, aggregation, metrics, and key
advancement active while syncing.
- Add tests covering gating, recovery, and network-wide stalls.

## Correctness / Behavior Guarantees

- Proposals and attestations are disabled only while the local node is
considered syncing.
- Duties resume once the node catches up past the hysteresis recovery
boundary.
- Duties remain enabled during a network-wide stall so validators can
help the chain recover.
- Incoming network data and synchronization processing remain
unaffected.
- A skipped proposal is not reported to fork choice as locally
available.

## Tests Added / Run

- Added tests verifying:
  - Proposals and attestations are gated while syncing.
  - Duties resume after catching up.
  - Duties remain enabled during network-wide stalls.
- Ran `cargo test -p ethlambda-blockchain --lib --offline`.
  - 31 passed; 0 failed.
- Ran `cargo clippy -p ethlambda-blockchain --lib --offline -- -D
warnings`.
- Ran `cargo fmt --all -- --check`.
- Ran `git diff --check`.

## Related Issues / PRs

- Closes lambdaclass#236
- Depends on the sync-status heuristic PR.
- Follows leanEthereum/leanSpec#708.

## ✅ Verification Checklist

- [x] Ran `make fmt` — clean
- [x] Ran `make lint` (clippy with `-D warnings`) — clean
- [ ] Ran `cargo test --workspace --release` — all passing

---------

Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>
Co-authored-by: Pablo Deymonnaz <pdeymon@fi.uba.ar>

Author: 3 people

crates/blockchain/src/lib.rs

@@ -96,6 +96,14 @@ impl SyncStatusTracker {
             metrics::SyncStatus::Synced
         }
     }
+
+    fn duties_allowed(&self) -> bool {
+        !self.syncing
+    }
+
+    fn gate_proposer(&self, proposer: Option<u64>) -> Option<u64> {
+        proposer.filter(|_| self.duties_allowed())
+    }
 }
 
 /// Milliseconds until the next interval boundary, measured relative to genesis.
@@ -273,9 +281,16 @@ impl BlockChainServer {
         // At interval 0, check if we will propose (but don't build the block yet).
         // Tick forkchoice first to accept attestations, then build the block
         // using the freshly-accepted attestations.
-        let proposer_validator_id = (interval == 0 && slot > 0)
+        let scheduled_proposer = (interval == 0 && slot > 0)
             .then(|| self.get_our_proposer(slot))
             .flatten();
+        let proposer_validator_id = self.sync_status.gate_proposer(scheduled_proposer);
+
+        if let Some(validator_id) = scheduled_proposer
+            && proposer_validator_id.is_none()
+        {
+            info!(%slot, %validator_id, "Skipping block proposal while syncing");
+        }
 
         // Snapshot the pre-merge `new_payloads` set at the end-of-slot promote
         // (interval 4), so the post-block report for this round sees its
@@ -334,7 +349,11 @@ impl BlockChainServer {
                     slot - 1,
                 );
             }
-            self.produce_attestations(slot, is_aggregator);
+            if self.sync_status.duties_allowed() {
+                self.produce_attestations(slot, is_aggregator);
+            } else if !self.key_manager.validator_ids().is_empty() {
+                info!(%slot, "Skipping attestations while syncing");
+            }
         }
 
         // Update safe target slot metric (updated by store.on_tick at interval 3)
@@ -1068,6 +1087,12 @@ mod tests {
                 metrics::SyncStatus::Synced
             );
         }
+
+        let first_syncing_slot = 10 + SYNC_LAG_THRESHOLD + 1;
+        assert_eq!(
+            tracker.update(first_syncing_slot, 10, first_syncing_slot),
+            metrics::SyncStatus::Syncing
+        );
     }
 
     #[test]
@@ -1112,4 +1137,32 @@ mod tests {
 
         assert_eq!(tracker.update(15, 20, 20), metrics::SyncStatus::Synced);
     }
+
+    #[test]
+    fn syncing_gates_proposals_and_attestations() {
+        let mut tracker = SyncStatusTracker::default();
+        tracker.update(20, 0, 20);
+
+        assert!(!tracker.duties_allowed());
+        assert_eq!(tracker.gate_proposer(Some(3)), None);
+    }
+
+    #[test]
+    fn caught_up_node_allows_proposals_and_attestations() {
+        let mut tracker = SyncStatusTracker::default();
+        tracker.update(20, 0, 20);
+        tracker.update(20, 18, 20);
+
+        assert!(tracker.duties_allowed());
+        assert_eq!(tracker.gate_proposer(Some(3)), Some(3));
+    }
+
+    #[test]
+    fn network_stall_keeps_proposals_and_attestations_enabled() {
+        let mut tracker = SyncStatusTracker::default();
+        tracker.update(100, 0, 0);
+
+        assert!(tracker.duties_allowed());
+        assert_eq!(tracker.gate_proposer(Some(3)), Some(3));
+    }
 }